At Friday's press conference Supreme Commander of the Swedish Armed Forces Sverker Göransson said that Sweden had "used all the available assets that we have in the royal navy" as part of its investigation, including using "different kinds of sensors".

But he refused to give further details of exactly how officials had confirmed the existence of the vessel, stating "that's top secret".

Warships, minesweepers, helicopters and more than 200 troops scoured an area about 30 to 60 kilometres (20 to 40 miles) from Stockholm in October following reports of a "man-made object" in the water, but the search was called off after the Swedish military said it believed that the suspect vessel had moved on.

The drama evoked memories of dozens of Cold War U-boat hunts in the 1980s.

In the most dramatic incident, a Soviet submarine U137 ran aground close to one of Sweden's largest naval bases in 1981 and was only allowed to leave after a humiliating wait.

Apart from that incident and several confirmed visual sightings, Sweden has never produced hard evidence of a Russian submarine in its waters.

Tomas Ries, an expert at the Swedish National Defence College said Friday's announcement was a major breakthrough for the Swedish military.

"During 10 years of obsessive Cold War submarine hunts, they never came up with anything, nothing at all," he told news agency TT.